In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and a light source with a shorter wavelength.
Resist materials adapted for KrF excimer laser (248 nm) started use on the 0.3 μm process and entered the mass production phase on the 0.13 μm rule. A wavelength change-over from KrF to shorter wavelength ArF excimer laser (193 nm) is expected to enable miniaturization of the design rule to below 0.13 μm. Since commonly used novolak resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, acrylic resins and alicyclic (typically cycloolefin) resins are investigated, leading to mass-scale production of devices by the ArF lithography.
For the next 45-nm node devices which required an advancement to reduce the wavelength of exposure light, the F2 lithography of 157 nm wavelength became a candidate. However, for the reasons that the projection lens uses a large amount of expensive CaF2 single crystal, the scanner thus becomes expensive, hard pellicles are introduced due to the extremely low durability of soft pellicles, the optical system must be accordingly altered, and the etch resistance of resist is low; the F2 lithography was postponed and instead, the early introduction of ArF immersion lithography was advocated. This enables mass-scale production of 45-nm node devices. For the mass-scale production of 32-nm node devices, the double patterning process utilizing sidewall spacer technology is used although the process suffers from complexity and length.
For the fabrication of 32-nm node and subsequent devices, the extreme ultraviolet (EUV) lithography using an exposure wavelength of 13.5 nm which is shorter than the conventional lasers by one order of magnitude and thus featuring improved resolution is expected rather than the double patterning process with noticeable costs. Efforts are focused on the EUV lithography.
In the EUV lithography, a low laser power and light attenuation by reflecting mirror lead to a reduced quantity of light. Then light with a low intensity reaches the wafer surface. It is urgently demanded to develop a high-sensitivity resist material in order to gain a throughput despite a low light quantity. However, a trade-off relationship of sensitivity is pointed out that the sensitivity of resist material can be increased at the sacrifice of resolution and edge roughness (LER, LWR).
EUV resist materials are susceptible to environmental impacts because of their high sensitivity. In general, an amine quencher is added to chemically amplified resist materials for rendering the resist materials unsusceptible to contamination with airborne amine. The amount of amine quencher added to the EUV resist materials is only a fraction as compared with the ArF and other lithography resist materials. As a result, the EUV resist materials tend to take a T-top profile under the influence of amine from the resist surface.
For shutting off environmental impacts, it is effective to form a protective film on top of the resist film. Application of a protective film was effective for the chemically amplified resist materials of the early stage based on a t-BOC-protected polyhydroxystyrene and free of an amine quencher, used in the KrF excimer laser lithography. Also at the initial stage of the ArF immersion lithography, a protective film was applied to prevent the acid generator from leaching out in water and thus inhibit the profile from becoming T-topped.
With respect to the EUV lithography, it was also proposed to form a protective film on top of the resist film (see Patent Documents 1 to 3 and Non-Patent Document 1). The protective film thus formed is effective for improving environmental resistance and reducing the outgassing from the resist film.
EUV laser sources of the discharge-produced plasma (DPP) or laser-produced plasma (LPP) method emit not only light of wavelength 13.5 nm available for pattern formation, but also broad light of wavelength 140 to 300 nm, known as out-of-band (OOB) light. The broad light has a low intensity, but a wide span of wavelength and is not negligible as energy quantity. The EUV microstepper is loaded with a Zr filter for cutting off OOB light, but the quantity of light is reduced thereby. The EUV scanner may not be loaded with the filter because a reduction of light quantity is not permissible for the goal of enhancing the throughput.
There is a need for a resist material which is sensitive to EUV, but not to OOB light. For such resist materials, the cation structure of sulfonium salt PAG is important. Patent Document 4 (JP-A 2011-138107, paragraph [0052]) describes a polymer-bound acid generator having a high sensitivity to EUV light, but a low sensitivity to OOB light. Non-Patent Document 1 describes the superiority of a protective film which is formed on top of the resist layer for cutting off OOB light.
In connection with a protective film for use in the immersion lithography, it is pointed out in Patent Document 5 that the solvent for the coating of a protective film dissolves the resist film surface to invite intermixing between the protective film and the resist film, resulting in a thickness loss of the resist pattern after development. The film thickness loss becomes outstanding particularly when alcohol solvents are used. Ether solvents are effective for preventing thickness loss. Polymers that are dissolved in ether solvents include polymers containing hexafluoroalcohol (HFA) as described in Patent Document 4. However, since fluorine atoms have strong absorption to EUV light, a resist film on which a protective film is formed using a HFA-containing polymer undesirably exhibits low sensitivity on patterning.